Pre-solar grains discovery helps in identifying recipe for the planets

Tiny mineral grains from meteorites are revealing the secrets of what ingredients went into the formation of our solar system 4.567 billion years ago.

The findings of an international collaboration including researcher Dr Bruce Charlier from Te Herenga Waka—Victoria University of Wellington in Aotearoa New Zealand have just been published in the journal Science Advances and challenge existing models of solar system formation at the chemical level.

Dr Charlier and his collaborators Assistant Professor François Tissot of the California Institute of Technology and Professor Nicolas Dauphas of the University of Chicago have demonstrated the complexity of primitive meteorites, such as the well-known and studied Allende meteorite, which fell to Earth in Mexico in February 1969.

Dr Charlier, who manages the Geochemistry Lab in Te Kura Tātai Aro Whenua—School of Geography, Environment and Earth Sciences, is leading a $624,000 Marsden Fund-supported study of the oldest components within “primitive” meteorites.

“Primitive meteorites are rare because they are the bits and pieces left flying around that haven’t been incorporated into planets,” he says. “So they hold clues about some of those very early processes.

“These primitive meteorites are made up of a number of different components that have different histories and are formed under different conditions.

“Some of the components are actually splashes of melt formed during impacts of asterioids, plus there’s also a lot of carbonaceous dust, but the oldest components in this mix are called calcium-aluminium inclusions (CAIs).

“It is widely thought these were condensed from a high-temperature gas as the Sun was forming. This makes them very different from minerals you might find in an igneous rock on Earth, which crystallised from a magma.”

In their paper, Dr Charlier and his colleagues describe evidence of pre-solar grains found within the CAIs themselves. This shows the process is more complex than previously thought.

Research into CAIs and meteorite composition is highly technical and detailed work, requiring specialist scientific instruments.

Dr Charlier and his team are splitting the CAIs into even smaller components. It is like the astronomical equivalent of the Russian Matroyoshka stacking dolls.

They deploy a chemical process called step-leaching to incrementally dissolve the components of the CAIs in different strengths of acid, and then study the different fractions using a thermal-ionisation mass spectrometer.

“Step-leaching is a little bit of a blunt instrument because you are not entirely sure what exactly it is you are destroying at each step. But the nub of what we’ve found is, once you have stripped away 99 percent of the common components within the CAIs, what we are left with is something highly exotic that we weren’t expecting.

“Those highly resistant materials are, we think, tiny pre-solar grains pre-dating our solar system, because when you measure their isotopic and chemical composition on the mass spectrometer they are very, very different to anything else you find.

“Pre-solar grains have been isolated from the matrix of primitive meteorites before. But what we have found is that pre-solar grains are preserved within the CAIs and are actually quite different.

“This is really interesting. We want to know what the nature of this material is and how it fits into the mix of ingredients that went to form the recipe for the planets.”

The pre-solar grains are aged somewhere between 4.567 billion years old, when the solar system was formed, and 13.8 billion years old, about the time of the Big Bang, and represent the vestiges of material left over from the birth of the solar system.

Dr Charlier has been at Te Herenga Waka—Victoria University of Wellington for five years and runs specialist analytical apparatus perfectly suited for meteorite research.

The laboratory is a leading facility in Aotearoa New Zealand for this type of research.

He has become one of the only technical staff members at the University to win a Marsden Fund grant and says the grant will enable the team to continue to investigate the pre-solar grains.

A PhD student and a Master’s student will also be involved in the project.